Make Server Options Configurable in RSpec Helper

* detect if a `chef_zero_opts` method is available (can be a regular method or RSpec let binding) and merge any given options with the defaults. This lets you bind chef-zero to IPv6 in tests.
* Build `ChefZero::Server`'s `url` attribute in an IPv6-safe manner: detect IPv6 by the presence of a colon character in the bind address and escape it by wrapping it in square brackets. Chef-zero works fine without this, but integration with other code (test code in particular) will not work on IPv6 without this.